1. Field of the Invention
The present invention relates to a method for forming a metal oxide film and a method for forming a secondary electron emission film in a gas discharge tube. More specifically, the present invention relates to a method for forming a metal oxide film on an inner wall of a tube.
2. Description of the Related Arts
A variety of methods for forming a metal oxide film on an inner wall of a tube have been conventionally known, and among these, for example, a method which comprises applying a coating solution containing an organic metal compound on the inner wall of the tube to form a coating film, and performing thermal treatment on the coating film, thereby forming the metal oxide film is commonly used.
The metal oxide film formed in the method as described above, however, has a problem that the thickness of the metal oxide film is not uniform. This problem becomes more significant in thin tubes having an inner diameter of not more than 2 mm.
This nonuniform thickness is ascribable to the fact that in performing the thermal treatment on the coating film containing the organic metal compound, when the temperature reaches the melting point of the organic metal compound, the viscosity of the coating film suddenly decreases so that the coating solution applied on the three-dimensional face drops down due to the gravity, making the thickness nonuniform, and this coating film is finally transformed into the metal oxide film. If the thickness of the metal oxide film becomes nonuniform, cracking in the film is induced, and partial variations may occur in characteristics requested for metal oxide film, such as secondary electron emission characteristic and resistance value.
For this reason, it has been desired to provide a method for forming a metal oxide film having a uniform thickness with respect to a three-dimensional surface such as inner wall of a tube, by using an organic metal compound which is to become a metal oxide film as a result of thermal treatment.